Lasers are commonly used in processes for separating glass plates. For example, U.S. Pat. No. 5,776,220 describes the use of a laser to propagate a blind crack along the surface of a glass sheet. To form the crack, the glass surface is subjected to a process of laser heating followed by rapid cooling. A crack made to score the glass in a glass sheet separation process is typically referred to as a median or vent crack. Because the crack is used to score the glass sheet, it typically extends only partway through the depth of the glass sheet. In this fashion, a glass sheet can be simply and cleanly separated into two smaller sheets by separation along the line of the median crack.
A median crack can be formed by making a small nick or scribe in one surface of the glass sheet. A laser beam is made to impinge on the glass sheet, beginning at the nick. The beam is then moved relative to the glass, generally at a speed between 200 and 700 millimeters per second. The laser beam is made to travel across the glass to trace the path of the scoreline. As the laser beam heats the surface of the glass, a stream of fluid coolant strikes a point just behind the laser beam, relative to the beam's motion across the glass surface. This process of heating followed by rapid cooling creates stresses in the glass sheet, which form a crack that extends along the line of motion of the laser and coolant.
When the scoring process is done rapidly, the thermal energy that forms the median crack is stored in a relatively thin region at the surface of the glass. By way of example, using a CO.sub.2 laser operating in D-Mode and running at a speed of 500 mm per second, most of the heat is contained within a region of less than 500 micrometers below the glass surface. This attribute of laser scoring permits the formation of cracks which only extend partway through the glass.
The "shape" of the electromagnetic field within the laser resonator is dependent upon the mirror curvature, spacing and bore diameter of the discharge tube and the energy's wavelength. The "shape" of the beam formed by the laser is generally classified according to the number of nulls that appear across the beam cross-section in two directions. For most purposes, a beam without nulls having a Gaussian power distribution is preferred. However, for glass separation processes, a non-Gaussian mode with one or more nulls can be used to deliver the laser energy more uniformly to the glass surface, resulting in higher effective laser scoring speeds.
A laser operating in D-mode is described in U.S. Pat. No. 5,776,220, which is incorporated by reference in its entirety. FIG. 2 illustrates a cross section of a D-mode laser beam's power distribution, in accordance with this invention. Such non-Gaussian beams, which have at least a pair of intensity peaks located outside a center region of lower power distribution, are preferred in the present invention.
As shown by Kondratenko (PCT WO 93/20015), the footprint shape for the laser beam as it impinges on the glass sheet may be elliptical. The minor and major axes of this elliptical footprint will typically satisfy the following relationship: EQU a=0.2 to 2.0 h,
and EQU b=1.0 to 10.0 h,
where a is the length of the minor axis and b is the length of the major axis; and h is the thickness of the glass sheet which is being laser scored. According to Kondratenko, when b is greater than 10.0 h, problems in the accuracy of the cutting process arise. Thus, for a glass substrate having a thickness of 0.7 mm (a common thickness for liquid crystal display substrates), Kondratenko teaches that the major axis of the beam spot should not exceed 7 mm in length.
To form an elliptical beam, the laser beam profile generated by the D-Mode is typically transformed with two cylindrical lenses to form a beam with an elliptical footprint. The elliptical beam is used to directly irradiate the glass surface. Using this technique, the depth of the median crack typically ranges from 115 to 118 micrometers with a 280 Watt beam or from 120 to 125 micrometers with a 330 Watt beam.
These laser-scoring techniques provide separation edges of a good quality, by forming a particle-free median crack. The reliability of the procedures and the resulting quality make laser scoring useful in the manufacture of liquid crystal and other flat panel display substrates, where the quality of edge breaks is desirably very high. Additionally almost any application which requires reforming sheet glass, such as the manufacture of auto windows, cosmetic mirrors, or residential windows can advantageously use laser scoring.
However, in some applications where handling of the glass parts is required, after the scoring has been made, but before the separation process, the handling of the scored glass may cause the parts to separate prematurely. One method of preventing this problem is to form shallow median cracks that are less susceptible to unintended separation. However, previously disclosed laser scoring methods do not permit such fine control of the median crack's depth, nor produce a nearly uniform score depth regardless of any change to the power of the laser beam, or the position of the cooling water stream.
Therefore, a need exists for a method of controlling the penetration depth of the median crack produced by a laser scoring technique.